Weight gain occurs in 60% or more of all breast cancer survivors, and this has been shown to jeopardize long-term survival. In healthy populations, past studies show that physical activity, particularly vigorous activity, and a diet high in take of fiber and low intake of trans-fats and refined carbohydrates may help women to maintain weight over time. It is however unclear whether a prudent lifestyle, despite the stress of breast cancer and side effects of treatment, can help women to maintain weight after cancer. We propose to conduct one of the largest prospective investigations of lifestyle factors with empirical support in the literature for predicting weight maintenance, and trajectory of weight after breast cancer diagnosis, compared with the weight trajectory of a population of women free of breast cancer. The analysis seeks to address how diet and activity may be used to manage weight after breast cancer diagnosis. Lifestyle factors to be explored include both physical activity (amount and intensity) and dietary factors (intake of total fiber, cereal fiber, trans-fat, refined starches and sugars, sugar-sweetened beverages), as well as the optimal combination of diet and activity. The sample population is comprised of a large cohort of women with (n=7,072) and without (n= 206,721) breast cancer from both the Nurses' Health Study (NHS) and NHS II cohorts ranging in age from 28 to 79 years. Analyses of pre-diagnosis and post-diagnosis diet and trajectory of weight post-diagnosis will be performed using longitudinal mixed linear models of repeated measures. This study may provide the background for an intervention of diet and physical activity that may help to prevent weight gain in breast cancer survivors. Unique strengths of the study include the ability to examine weight at several time points and to compare to a population of women free of breast cancer. These data furthermore provide a unique opportunity to study lifestyle influences across a wide age range, and enable exploration by factors including baseline weight, menopausal status, and treatment.